21 (21q(22) Trisomy 21 - Down Syndrome
Down syndrome is caused by an extra chromosome 21. It is the most common single cause of human birth defects, with an occurrence in 1 out of every 660 births. Congenital heart defects are frequently present in Down syndrome children. The normal life span mainly is shortened in Down syndrome by congenital heart disease and by increased incidence of acute leukemia. Mental retardation is variable, and usually moderate. Some adults live independently and are accomplished individuals.